1. Field of the Invention
The present invention relates to nanoporous dielectric films and to a process for their manufacture. Such films are useful in the production of integrated circuits.
2. Description of the Prior Art
As feature sizes in the production of integrated circuits approach 0.25 .mu.m and below, problems with interconnect RC delay, power consumption and crosstalk all become more significant. Integration of low dielectric constant materials for interlevel dielectric (ILD) and intermetal dielectric (IMD) applications partially mitigate these problems, however each of the material candidates having K significantly lower than dense silica suffer from disadvantages. A number of organic and inorganic polymers have K in the range of 2.2 to 3. 5, however, these polymers suffer from problems including low thermal stability, poor mechanical properties including low glass transition temperature (Tg), sample outgassing, and long term reliability questions. One alternative is to employ nanoporous silicas which can have dielectric constants in the range of about 1 to 3. Nanoporous silica is attractive because it employs similar precursors (e.g., TEOS, tetraethoxysilane) as used for spin-on glasses (SOG's) and CVD SiO.sub.2 and because of the ability to control pore size of the nanoporous silica. In addition to low dielectric constant, nanoporous silica offers other advantages for microelectronics including thermal stability up to 900.degree. C., small pore size (&lt;&lt;microelectronics features), use of materials that are widely used in the semiconductor industry namely silica and precursors (e.g., TEOS), the ability to tune dielectric constant over a wide range, and deposition using similar tools as employed for conventional SOG processing.
Nanoporous silica films can be fabricated by using a mixture of a solvent composition and a silica precursor which is deposited onto a wafer by conventional methods of spincoating, dip-coating, etc. The silica precursor is polymerized by chemical and/or thermal means until it forms a gel. Film thickness and density/dielectric constant can be controlled independently by using a mixture of two solvents with significantly different volatility. The more volatile solvent evaporates during and immediately after precursor deposition. The second solvent is then removed by increasing the temperature. EP patent application EP 0 775 669 A2, which is incorporated herein by reference, shows a method for producing a nanoporous silica film with uniform density throughout the film thickness.
Nanoporous silicas are preferably prepared from precursors comprising an alkoxysilane, a relatively high volatility solvent and a relatively low volatility solvent which is a polyol having an ether linkage. The principal reactions for an alkoxysilane such as tetraethoxysilane, tetramethoxysilane, triethoxysilane, methyltriethoxysilane, colloidal silica, and silica precursors which contain silicon-organic-silicon linkages are shown below. The following are exemplary reactions since the extent of hydrolysis and transesterification can vary from 0 to 4.
1) Hydrolysis: Si(OR).sub.4 +H.sub.2 O&lt;--&gt;Si(OR).sub.3 OH+ROH PA0 2) Transesterification: Si(OR).sub.4 +R"OH&lt;--&gt;Si(OR).sub.3 OR"+ROH PA0 3) Water condensation: Si(OR).sub.3 OH+Si(OR).sub.3 OH&lt;--&gt;Si(OR).sub.3 OSi(OR).sub.3 +H.sub.2 O PA0 4) Alcohol condensation: Si(OR).sub.3 OH+Si(OR).sub.4 &lt;--&gt;Si(OR).sub.3 OSi(OR).sub.3 +ROH PA0 a) blending a nanoporous silica precursor composition which comprises at least one alkoxysilane; at least one relatively low volatility solvent composition comprising a linear or branched C.sub.1 to C.sub.4 alkyl ether of a C.sub.1 to C.sub.4 alkylene glycol which is miscible in water and alkoxysilanes, having a hydroxyl concentration of 0.0084 mole/cm.sup.3 or less, a boiling point of about 175.degree. C. or more at atmospheric pressure and a weight average molecular weight of about 120 or more; at least one relatively high volatility solvent composition having a boiling point below that of the relatively low volatility solvent composition; optional water and an optional catalytic amount of an acid, thus forming a mixture and causing a partial hydrolysis and partial condensation of the alkoxysilane; PA0 b) depositing the composition onto a substrate while evaporating at least a portion of the relatively high volatility solvent composition; PA0 c) exposing the composition to a water vapor and a base vapor; and PA0 d) evaporating the relatively low volatility solvent composition, thereby forming a relatively high porosity, low dielectric constant, silicon containing polymer composition on the substrate. PA0 a) blending a nanoporous silica precursor composition which comprises at least one alkoxysilane; at least one relatively low volatility solvent composition comprising a linear or branched C.sub.1 to C.sub.4 alkyl ether of a C.sub.1 to C.sub.4 alkylene glycol which is miscible in water and alkoxysilanes, having a hydroxyl concentration of 0.0084 mole/cm.sup.3 or less, a boiling point of about 175.degree. C. or more at atmospheric pressure and a weight average molecular weight of about 120 or more; at least one relatively high volatility solvent composition having a boiling point below that of the relatively low volatility solvent composition; optional water and an optional catalytic amount of an acid, thus forming a mixture and causing a partial hydrolysis and partial condensation of the alkoxysilane; PA0 b) depositing the composition onto a semiconductor substrate while evaporating at least a portion of the relatively high volatility solvent composition; PA0 c) exposing the composition to a water vapor and a base vapor; and PA0 d) evaporating the relatively low volatility solvent composition, thereby forming a relatively high porosity, low dielectric constant, silicon containing polymer composition on the semiconductor substrate.
Thus, to completely hydrolyze and condense a tetrafunctional alkoxysilane such as TEOS, two moles of water are required per mole of silane. Typically, the precursor is prepared with an insufficient quantity of water so that stability is maintained during transportation and storage. After deposition, additional water is absorbed into the film and reactions may go to the desired completion. The problem is that the surface chemistry and molecular weight of the precursor changes dramatically during deposition and post-processing. During the initial deposition, the silicon polymer is primarily covered with alkoxy groups which are immiscible with water. At the same time, the composition of the solvent system is changing dramatically as the high volatility solvent (typically, an alcohol such as ethanol or isopropanol) rapidly evaporates during deposition. During the initial deposition, the silicon precursor polymer is primarily covered with alkoxy groups which are immiscible with water. However, after exposure to additional water and a catalyst, the surface rapidly changes to a hydrophilic surface primarily covered with silanol groups (SiOH). In order to avoid the possibility of microphase separation in the gel which can result in the presence of defects in the film and film appearance problems such as hazing and streaks, we have recently discovered that the low volatility pore control solvent (PCS) must have a set of properties which were not heretofore known.
It has now been found that the low volatility polyol solvent must meet a number of criteria which were previously not known in order to achieve a stable precursor solution. In addition to having a high boiling point and proper solubility in water and alkoxysilanes, important criteria are low hydroxyl concentration and high molecular weight. In order to obtain films of high quality, i.e. having no defects, no large pores, etc., it has been discovered that the silane precursor and water should be miscible in the pore control solvent. During precursor synthesis, the components are typically made mutually miscible via the high volatility solvent. However, this invention is predicated on these components being miscible in the PCS after the high volatility solvent has evaporated. In order to obtain precursor solutions which are stable over long time (i.e. months) and which will give nanoporous silica films with desirable properties such as high surface area, high mechanical strength, and small pore size, the solvents need to be employed with volumetric hydroxyl concentrations in the correct range. The desired range of hydroxyl concentration depends upon the target dielectric constant.
According to the invention one group of solvents that are uniquely suited to meeting each of these constraints are the monomethyl ethers of ethylene glycol and propylene glycol. The compounds have ether linkages (C--O--C) and alcohol groups (COH). A single compound may be used or a mixture of compounds may be employed to achieve the desired properties. When a pore control solvent is used which does not exhibit adequate miscibility with both the alkoxysilane and water, defects in the film are observed. Scanning electron micrographs of nanoporous silica films produced using either a PCS (e.g., tetraethylene glycol) which has appropriate properties such as boiling point and hydroxyl content but which is not miscible with TEOS and made using a PCS (e.g., triethylene glycol monomethyl ether), which is the subject of this invention and which is miscible with both TEOS and water, demonstrate that the defects which can arise as a result of miscibility problems are readily apparent for the film made with the tetraethylene glycol, but not with the film made with the triethylene glycol monomethyl ether.